Low NOx emissions from a gas turbine engine, of below 10 volume parts per million (ppmv), are becoming important criteria in the selection of gas turbine engines for power plant applications. Some installations in non-attainment area in the United States are demanding even lower NOx emissions of less than 5 ppmv. The challenging NOx emission requirements must be achieved without compromising the more conventional constraints on gas turbine engines, of durability, low operating costs and high efficiency.
The main factor governing nitrogen oxide formation is temperature. One of the most attractive methods of reducing flame temperatures involves using Lean Premixed combustion, in which reductions in flame temperatures are readily accomplished by increasing the air content in a given fuel/air mixture. This method is often referred to as a Dry-Low-Emissions (DLE) to distinguish it from Wet NOx control by water or steam injection, and highlight the low emissions in which NOx levels down to 10 ppmv can be achieved.
However, flame stability decreases rapidly under the lean combustion conditions and the combustor may be operating close to its blow-out limit. In addition, severe constraints are imposed on the homogeneity of the fuel/air mixture since leaner than average pockets of mixture may lead to stability problems and richer than average pockets will lead to unacceptably high NOx emissions. The emission of carbon monoxide as a tracer for combustion efficiency will increase at leaner mixtures for a given combustor due to the exponential decrease in chemical reaction kinetics. Engine reliability and durability are of major concern under lean combustion conditions due to high-pressure fluctuations enforced by flame instabilities in the combustor.
It is well known in the industry that catalytic combustion can be used as an ultra-lean premixed combustion process where a catalyst is used to initiate and promote chemical reactions in a premixed fuel/air mixture beyond flammability limits that would otherwise not burn. This permits a reduction of peak combustion temperatures to levels below 1,650K, and NOx emissions less than 5 ppmv can be achieved.
Nevertheless, major challenges have prevented the implementation of catalytic combustors in a gas turbine engine. Catalyst operation and durability demand a very tight control over the engine and catalyst inlet operating parameters. As shown in FIG. 1, which is a graphical representation of a normalized catalyst operating window and the compressor discharge temperature variations from engine idle to full power, the compressor discharge temperature increase from engine idle to full power over a range typically more than three times that which, as being defined between lines M and N, is acceptable for catalyst operation.
In the prior art, most Catalyst combustion systems utilize a pre-burner to increase compressor discharge air temperature at engine low power conditions where the compressor discharge air temperature is below catalyst ignition temperature. Other major problems in catalyst operation include ignition, engine start-up and catalyst warm up which cannot be performed with the catalyst. A separate fuel system is required. Any liquid fuel combustion has to be introduced downstream of the catalyst to prevent liquid fuel flooding the catalyst in case of ignition failure. Because of the narrow range of acceptable catalyst inlet temperatures, the catalyst has to be designed for full power operating conditions. As the engine decelerates the fuel/air mass ratio decreases. Generally, this compromises the catalyst and engine performance under part load conditions, thereby resulting in emissions leading to very high NOx and CO levels. The catalyst durability is affected by engine transient operation since catalyst operation is a delicate balancing act between catalyst ignition (blow-out) and catalyst burn-out. In this sense, turn-down of the catalyst system becomes a serious operability and durability issue. In the case when the pre-burner is used for part load of the entire operating range of the engine, the pre-burner then becomes the main source of NOx emissions from the engine. In addition, hot streaks from the pre-burner are very likely to damage catalyst hardware directly or act as sources of auto-ignition within the fuel/air mixing duct upstream of the catalyst, and impose a substantial risk to catalyst and engine operation. A pre-burner also substantially increases the combustor pressure drop by an additional 1.5% to 2.5%, which directly affects engine specific fuel consumption.
Efforts have bean made to improve catalytic combustors for gas turbine engines. One example of the improvements is described in U.S. Pat. No. 5,623,819, issued to Bowker et al. on Apr. 29, 1997. Bowker et al. describe a low NOx generating combustor in which a first lean mixture of fuel and air is pre-heated by transferring heat from hot gas discharging from the combustor. The pre-heated first fuel/air mixture is then catalyzed in a catalytic reactor and then combusted so as to produce a hot gas having a temperature in excess of the ignition temperature of the fuel. Second and third lean mixtures of fuel and air are then sequentially introduced into the hot gas, thereby raising their temperatures above the ignition temperature and causing homogeneous combustion of the second and third fuel/air mixtures. This homogeneous combustion is enhanced by the presence of the free radicals created during the catalyzing of the first fuel/air mixture. In addition, the catalytic reactor acts as a pilot that imparts stability to the combustion of the lean second and third fuel/air mixtures.
Another example of the improvements is described in U.S. Pat. No. 5,050,731, issued to Beebe et al. on Dec. 22, 1998. Beebe et al describe a combustor for gas turbine engines and a method of operating the combustor under low, mid-range and high load conditions. At the start-up or low-load levels, fuel and compressor discharge air are supplied to the diffusion flame combustion zone to provide combustion products for the turbine. At mid-range operating conditions, the products of combustion from the diffusion flame combustion zone are mixed with additional hydrocarbon fuel for combustion in the presence of a catalyst in the catalytic combustion zone. Because the fuel air mixture in the catalytic reactor bed is lean, the combustion reaction temperature is too low to produce thermal NOx. Under high-load conditions a lean direct injection of fuel/air is provided in a post-catalytic combustion zone where auto-ignition occur with the reactions going to completion in the transition between the combustor and turbine sections. In the post-catalytic combustion zone, the combustion temperature is low and the residence time in the transition piece is short, hence minimizing thermal NOx.
Nevertheless, there is still a need for further improvements of low emissions combustors for gas turbine engines that will allow minimizing the emissions of the NOx, CO and unburned hydrocarbon (UHC) simultaneously, over the entire operating range of the gas turbine engine.